1. Field of the Invention
This invention relates to headboxes which are used in papermaking machines. Such structures of this type, generally, employ the use of a venturi located between the headbox and the slice.
2. Description of the Related Art
It is known, that the approach system to the headbox for papermaking machines makes use of a system which mixes the pulp and the white water such that the pulp and white water mixture is substantially homogeneous. Exemplary of such prior art are U.S. Pat. Nos. 3,061,008 to C. W. E. Walker, 3,945,882 to D. Egelhof et al., and 4,897,160 to K. Wolf et al. While these systems can produce a mixture of pulp and white water, due to the fact that the pulp and white water are mixed long before the mixture exits the slice, the randomness of the pulp fibers within the mixture is substantially decreased. If the randomness of the pulp fibers within the mixture is decreased, the cross direction (CD) strength properties of the paper produced by this mixture is adversely affected. Therefore, an advantageous headbox would be one which maintained the randomness of the pulp fibers within the pulp/white water mixture.
It is also known, in papermaking headboxes, to make use of an air pad that is located within the headbox. The air pad, along with the flow and consistency regulation of the pulp/white water mixture, are used to regulate the caliper and basis weight of the sheet formed by papermaking machines. Also, the slice opening can be used to regulate the basis weight and caliper of the formed sheet. However, because the basis weight control valve of the pulp/white water mixture is located at a distance from the area where the pulp/white water mixture is placed upon the forming screen, the operator must allow a certain amount of time between corrections in the operating parameters in order to regulate the formation of the sheet. Therefore, a more advantageous headbox would be one which would allow the operator to more quickly correct the operating parameters of the headbox during the formation of the sheet.
Finally, it is known in the art to make use of a series of rectifier rollers located within the headbox in order to adequately mix the pulp/white water mixture. While this system adequately mixes the pulp/white water mixture and produces a homogeneous mixture, this system suffers from several defects. In particular, as discussed earlier, this system, as flow rates change, allows the randomness of the pulp fibers within the mixture to become decreased. Also, this system allows flocculations to be formed in the mixture. If these flocculations are not removed from the mixture, the flocculations remain in the sheet while the sheet is being formed and adversely affect the quality of the sheet. Therefore, a still further advantageous system would be one which avoids the use of rollers and the formation of flocculations.
It is apparent from the above that there exists a need in the art for a headbox which is capable of producing a homogeneous mixture of pulp and white water, and which can be adjusted more quickly in response to changes in forming characteristics of the paper sheet, but which at the same time would allow the pulp fibers within the pulp/white water mixture to maintain their randomness within the mixture. It is the purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.